Magneto-resistance sensors that are based on the GMR (Giant Magneto-Resistance) effect (P. Grünberg, R. Schreiber, Y. Pang, M. B. Brodsky, H. Sowers, Physical Review Letters 57, 2442 [1986]: “Layered Magnetic Structures: Evidence of antiferromagnetc coupling of Fe-Layers across Cr-interlayers”) illustratively are used as angular sensors or read heads in hard disk drives. In general however only the very high sensitivity to magnetic fields is being exploited for such purposes. Attempts already have been made to use so-called TMR (Tunnel Magneto-Resistance) elements as non-volatile magnetic storage media (MRAM, magnetic random access memory). The principles involves are summarily discussed below. A conventional, voltage-insensitive GMR sensor is configured as follows:
In the simplest case, two magnetic layers illustratively cobalt layers are separated by a non-magnetic layer, for instance made of copper. At the proper spacer layer thickness, the magnetic layers will couple anti-ferromagnetically as long as the external field is zero. If an external magnetic field is applied, the direction of magnetization of the magnetically softer layer will be rotated. At saturation, the two magnetic layers will couple parallel to each other. An electrical resistance differential arises between these two states.
This relative change in resistance, which is caused by an angular change of the directions of magnetization, is described by the relationΔR/R=(ΔR/2R)max(1−cos α)where (ΔR/R)max denotes the maximum relative change in resistance of a given system of layers and where α denotes the angle between the two directions of magnetization of the two magnetic layers.
Moreover there are layer configurations for which the magnetic layers remain uncoupled on account of a more substantial thickness of the spacer layer. The lower layer consists of a hard-magnetic material exhibiting uni-axial anisotropy pointing anti-parallel to the orientation of the soft-magnetic layer. This condition is typically attained using as the lower layer a magnetic layer coupled to a synthetic anti ferro-magnet and a soft-magnetic layer as the upper layer, said upper layer being rotatable by an external magnetic field. In the case of GMR, so-called “spring valves” are used. Relative resistivity changes ΔR/R of 3% to a maximum of 5% have been measured at room temperature for such configurations. Substantially higher values may be attained with multi-layer systems.
Basically the TMR structures exhibit a similar behavior as the GMR components. They are characterized in that the two magnetic electrodes are separated by a thin oxide barrier instead of a metallic, non-magnetic spacer layer. The tunnel current through the barrier depends on the directions of the electrode magnetizations as long as spin-flip dispersion is averted.
Isotropic ferro-magnets exhibit a magneto-elastic energy density described byEme=−(3σλs cos2θ)/2where λs is the magnetostriction at saturation and where σ is the external mechanical stress. This energy density describes the interaction between the magnetic torques and the internal and external mechanical stresses. The angle between the stress axis and the direction of magnetization is denoted by θ.
As regards positively magnetostrictive material under mechanical tension, it follows that the torques align in the direction of the axis of tension. Compressive stresses cause orientation perpendicularly to the stress axis. This behavior is reversed for negatively magnetostrictive materials.
The ratio of the magneto-elastic energy Eme to the total energy E is denoted by the magneto-mechanical coupling coefficient k33. This coefficient is defined as follows:k33=Eme/Etot.
The elongation sensitivityGF=(ΔR/R)/Δε  [gauge factor]i.e. the gains for the metal-based strain gauges are between 2 and 4. The so-called piezo-resistive sensors based on doped silicon are between 80 and 180.
Already a substantial number of magneto-resistance sensors using magnetostrictive materials is known. Illustratively U.S. Pat. No. 5,168,760 discloses a magnetic multi-layer exhibiting a periodic sequence of two different layers, one ferromagnetic, the other non-ferromagnetic. The ferromagnetic layers always couple to each other in anti-parallel manner. By applying a small magnetic field, the anti-ferromagnetic coupling of the layer torques is slightly changed toward ferromagnetic coupling. If magnetostrictive layers are used as the ferromagnetic layers, then an external mechanical stress may entail further rotation of the magnetic torques toward ferromagnetic coupling, resulting in a large change in resistance.
Moreover a two-element sensor based on the GMR effect is known whereby the effect of mechanical stress and magnetic field on the sensor signal may be separated. One hard-magnetic layer with a given direction of magnetization is used in both sensor elements and furthermore two soft-magnetic layers each time are separated by a non-magnetic one. These soft-magnetic layers are exposed to an oppositely directed magnetic biasing field of the same intensity. As a result the above cited separation of sensor signals may be attained by analyzing the sum and difference signal.
The following design also is known: a sensor consists of a pinned magnetic layer, of a non-magnetic layer and a free magnetostrictive layer in a configuration exhibiting the high magnetoresistive effect. This design exploits the fact that the permeability of the free magnetic layers changes on account of magnetostriction. When an appropriate magnetic biasing field is applied, a mechanical stress will entail a strong change in electrical resistance.
U.S. Pat. No. 5,856,617 describes a GMR layer configuration to measure the deflection of an AFM (atomic force microscope) cantilever beam. The magnetically free layer of this configuration exhibits non-vanishing magnetostriction. This document discloses a GMR layer configuration having a magnetostrictive structure composed of a triple layer of Ni—Fe, Ni and Co, and further applications as an AFM sensor.
Another publication has disclosed the effect of amorphous CoFeNiSiB layers acting as the magnetically soft layer in TMR elements. This alloy substance is a non-magnetostrictive alloy. This research led to observed TMR effects of 12%.